Cephalodium

In an even smaller number of cases – estimated at between 2 and 4 percent of all lichens – species of both a green alga and a cyanobacterium serve as photosynthetic partners.

[5] Most external cephalodia are gall-like structures that are simple in shape: generally rounded, lobed or sac-like.

[6] They can range in size from "minute" to several millimeters across and are typically a different color than the rest of the lichen's thallus.

[7] The fungal partner forms a thick layer of cortical tissue around the outside of each internal or external cephalodium, which helps to reduce the oxygen levels in the structure's interior.

[11] Many species of cyanobacteria grow epiphytically in the environment, where they may come into contact with the upper or lower surface of a lichen.

[13] Lichens containing cyanobacteria (cyanolichens) are among the major contributors of fixed nitrogen to some ecosystems, including mature tropical highland forests.